Electronic devices (e.g., cell phones, laptop computers, tablet computers, etc.) may use flash memory to store data. Flash memory is a type of non-volatile memory that retains stored data even when no power is applied to the memory. Over time, data errors may occur in flash memory due to various factors, including aging, temperature fluctuations, charge loss, and so on. Refresh operations are performed on the flash memory to correct these data errors. In flash memory, a refresh operation involves reading all the data from one particular memory block, correcting any errors in the read data, and then rewriting the corrected data (e.g., back into the same memory block or a different memory block).
One type of data error that may occur in flash memory is a bit-flip error (shown in Table 1, below) is an unintentional state switch (e.g., from 0 to 1, or from 1 to 0) for a particular memory bit. In NAND flash memory, bit-flip errors may occur due to manufacturing defects during ionizations, high strain on memory cells caused by read operations, and/or charge loss.
TABLE 1Bit-Flip Error ExampleAddressCorrupted DataLocationCorrect Data ByteByteRemarks0x00000x020x002nd data bit (from (binary = 0000 0010)(binary = 0000 0000)right) is flipped from 1 to 00x00800xB40x803rd, 5th and 6th bits (binary = 1011 0100)(binary = 1000 0000)(from right) areflipped from 1 to 0
Another type of data error that may occur in flash memory is a transmission error, which is an error that occurs in association with the communication of data between the flash memory and a host controller. An error correction code (ECC) process is conventionally used to address transmission errors. The ECC process includes adding redundant data, or parity data, to a message, such that the message can be recovered by a receiver (host controller or flash memory) even when a number of errors (up to a particular error threshold that is based on a capability of the ECC being used) is introduced, either during the process of transmission or on storage.